A digitizer board comprises a substrate carrying two groups of orthogonally disposed grid wires, each group comprising a plurality of spaced, parallel conductors. A selectively operable control circuit causes identical current pulses to flow in predetermined sequence and direction through the conductors of a group of grid wires for locating a measuring sensor positioned in inductive relationship with the digitizer board. The grid wires produce magnetic flux lines that are coupled to a coil in the measuring sensor. The resultant induced voltage in the coil is processed and provides the position of the coil relative to the board.
One important application of digitizers of the type described is to obtain the digital coordinates of individual points defining a two dimensional profile lying on the digitizer board. Proper utilization of the coordinates requires that no point on the profile should be digitized twice, and that no point on the profile should be omitted by oversight.
Conventionally, an operator places a sheet on which the profile is drawn over the digitizer board, and then positions the sensor at a selected point on the profile. By operating the control circuit, the operator is supplied with the coordinates of the point. To keep track of the process, the operator places a mark, with a pencil, for example, on the sheet at the selected point. This makes the procedure for obtaining the coordinates of the profile more difficult and time consuming than is desired; and a relatively large number of mistakes are likely to result.
An object of the invention, therefore, is to provide a new and improved marking device of the type described which facilitates marking selected points on a profile located on a digitizer board.